U.S. patent application number 15/339287 was filed with the patent office on 2017-02-16 for fixing apparatus that controls current for driving an induction heater.
The applicant listed for this patent is KABUSHIKI KAISHA TOSHIBA, TOSHIBA TEC KABUSHIKI KAISHA. Invention is credited to Yuki KAWASHIMA, Shuji YOKOYAMA.
Application Number | 20170045847 15/339287 |
Document ID | / |
Family ID | 54695496 |
Filed Date | 2017-02-16 |
United States Patent
Application |
20170045847 |
Kind Code |
A1 |
YOKOYAMA; Shuji ; et
al. |
February 16, 2017 |
FIXING APPARATUS THAT CONTROLS CURRENT FOR DRIVING AN INDUCTION
HEATER
Abstract
A fixing apparatus includes a belt including a ferromagnetic
layer. A ferromagnetic plate is disposed inside the belt and has a
Curie point that is lower than a Curie point of the ferromagnetic
layer. An induction heater causes heat generation in the
ferromagnetic layer and the ferromagnetic plate. The induction
heater includes a coil. A driving circuit outputs a high frequency
current to the coil, and changes the high frequency current. A
temperature sensor measures a temperature of the coil. A controller
controls the driving circuit to decrease the high frequency current
if the temperature of the coil measured by the temperature sensor
is higher than a predetermined value.
Inventors: |
YOKOYAMA; Shuji; (Sunto
Shizuoka, JP) ; KAWASHIMA; Yuki; (Tagata Shizuoka,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA TOSHIBA
TOSHIBA TEC KABUSHIKI KAISHA |
Tokyo
Tokyo |
|
JP
JP |
|
|
Family ID: |
54695496 |
Appl. No.: |
15/339287 |
Filed: |
October 31, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14927207 |
Oct 29, 2015 |
9501014 |
|
|
15339287 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03G 2215/2016 20130101;
G03G 15/2039 20130101; G03G 15/2053 20130101 |
International
Class: |
G03G 15/20 20060101
G03G015/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 2014 |
JP |
2014-240105 |
Jun 11, 2015 |
JP |
2015-118445 |
Claims
1. A fixing apparatus, comprising: a belt including a ferromagnetic
layer; a ferromagnetic plate disposed inside the belt and having a
Curie point that is lower than a Curie point of the ferromagnetic
layer; an induction heater configured to cause heat generation in
the ferromagnetic layer and the ferromagnetic plate, the induction
heater including a coil; a driving circuit configured to output a
high frequency current to the coil, and to change the high
frequency current; a temperature sensor configured to measure a
temperature of the coil; and a controller configured to control the
driving circuit to decrease the high frequency current if the
temperature of the coil measured by the temperature sensor is
higher than a predetermined value.
2. The fixing apparatus according to claim 1, wherein the
predetermined value is the Curie point of the ferromagnetic
plate.
3. The fixing apparatus according to claim 1, wherein the coil is
at a position corresponding to a sheet passing region of the belt
in a width direction of the belt.
4. The fixing apparatus according to claim 1, wherein the driving
circuit comprises switching elements including an insulated gate
bipolar transistor (IGBT) and a metal oxide semiconductor field
effect transistor (MOSFET), and the controller decreases the high
frequency current by extending an on period of the MOSFET.
5. A fixing apparatus, comprising: a belt including a ferromagnetic
layer; a ferromagnetic plate disposed inside the belt and having a
Curie point that is lower than a Curie point of the ferromagnetic
layer; an induction heater configured to cause heat generation in
the ferromagnetic layer and the ferromagnetic plate, the induction
heater including a coil; a driving circuit configured to output a
high frequency current to the coil, and to change the high
frequency current; a measurement unit configured to output a signal
indicative of whether a temperature of the ferromagnetic plate
exceeds a predetermined value; and a controller configured to:
receive the signal output from the measurement unit, determine
whether the temperature of the ferromagnetic plate exceeds the
predetermined value based on the signal, and control the driving
circuit to decrease the high frequency current if the temperature
of the coil exceeds the predetermined value.
6. The fixing apparatus according to claim 5, wherein the
predetermined value is the Curie point of the ferromagnetic
plate.
7. The fixing apparatus according to claim 5, wherein the coil is
at a position corresponding to a sheet passing region of the belt
in a width direction of the belt.
8. The fixing apparatus according to claim 5, wherein the driving
circuit comprises switching elements including an insulated gate
bipolar transistor (IGBT) and a metal oxide semiconductor field
effect transistor (MOSFET), and the controller decreases the high
frequency current by extending an on period of the MOSFET.
9. The fixing apparatus according to claim 5, wherein the
measurement unit is a temperature sensor.
10. The fixing apparatus according to claim 5, wherein the
measurement unit comprises: a second coil positioned proximate to
the ferromagnetic plate, and an electrical resistance measurement
circuit configured to measure an electrical resistance of the
second coil, the electrical resistance of the second coil
decreasing as the temperature of the second coil increases.
11. The fixing apparatus according to claim 5, wherein the
measurement unit comprises an electrical resistance measurement
circuit configured to measure an electrical resistance of the coil,
the electrical resistance of the coil decreasing as the temperature
of the coil increases.
12. A fixing apparatus, comprising: a belt including a
ferromagnetic layer; a ferromagnetic plate disposed inside the belt
and having a Curie point that is lower than a Curie point of the
ferromagnetic layer; an induction heater configured to cause heat
generation in the ferromagnetic layer and the ferromagnetic plate,
the induction heater including a coil; a driving circuit configured
to output a high frequency current to the coil, and to change the
high frequency current by switching on and off switching elements;
a measurement unit configured to output a signal indicative of
whether a temperature of the ferromagnetic plate exceeds a
predetermined value; and a controller configured to: receive the
signal output from the measurement unit, determine whether the
temperature of the ferromagnetic plate exceeds the predetermined
value based on the signal, and control the driving circuit to
decrease the high frequency current if the temperature of the coil
exceeds the predetermined value.
13. The fixing apparatus according to claim 12, wherein the
predetermined value is the Curie point of the ferromagnetic
plate.
14. The fixing apparatus according to claim 12, wherein the coil is
at a position corresponding to a sheet passing region of the belt
in a width direction of the belt.
15. The fixing apparatus according to claim 12, wherein the driving
circuit comprises switching elements including an insulated gate
bipolar transistor (IGBT) and a metal oxide semiconductor field
effect transistor (MOSFET), and the controller decreases the high
frequency current by extending an on period of the MOSFET.
16. The fixing apparatus according to claim 12, wherein the
measurement unit is a temperature sensor.
17. The fixing apparatus according to claim 12, wherein the
measurement unit comprises: a second coil positioned proximate to
the ferromagnetic plate, and an electrical resistance measurement
circuit configured to measure an electrical resistance of the
second coil, the electrical resistance of the second coil
decreasing as the temperature of the second coil increases.
18. The fixing apparatus according to claim 12, wherein the
measurement unit comprises an electrical resistance measurement
circuit configured to measure an electrical resistance of the coil,
the electrical resistance of the coil decreasing as the temperature
of the coil increases.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
patent application Ser. No. 14/927,207, filed on Oct. 29, 2015,
which is based upon and claims the benefit of priorities from
Japanese Patent Application No. 2014-240105, filed on Nov. 27, 2014
and Japanese Patent Application No. 2015-118445, filed on Jun. 11,
2015; the entire contents of each of the applications are
incorporated herein by reference.
FIELD
[0002] Embodiments described herein relate to a fixing apparatus,
in particular, a fixing apparatus that controls the current for
driving an induction heater.
BACKGROUND
[0003] An image forming apparatus such as a Multi-functional
Peripheral (hereinafter referred to as "MFP"), a printer, and the
like typically includes a fixing apparatus. The fixing apparatus of
one type causes heat generation for the fixing by an
electromagnetic induction heating unit (hereinafter referred to as
an "IH" unit). A fixing apparatus that includes the IH unit
includes a fixing belt and an auxiliary heating unit that generate
heat. The IH unit is usually configured to maintain its output
level to maintain a certain amount of heat generation. For example,
when the auxiliary heating unit loses its magnetism as the
temperature thereof increases too much, electric resistance of the
IH unit decreases. In this case, to maintain the output level,
level of a current supplied from a driving circuit to the IH unit
is increased. However, this increase of the current level may
damage the driving circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is a side view of an image forming apparatus
according to a first embodiment.
[0005] FIG. 2 is a side view of an IH coil unit and illustrates a
control block of a main control circuit according to the first
embodiment.
[0006] FIG. 3 is a perspective view of the IH coil unit according
to the first embodiment.
[0007] FIG. 4 illustrates magnetic paths of magnetic fluxes
extending along a fixing belt and an auxiliary heating plate
according to the first embodiment.
[0008] FIG. 5 is a block diagram of a control system, which
controls the IH coil unit according to the first embodiment.
[0009] FIG. 6 is a side view of a fixing apparatus according to the
first embodiment.
[0010] FIG. 7 is a flowchart illustrating an operation carried out
by the fixing apparatus according to the first embodiment.
[0011] FIG. 8 is a side view of a fixing apparatus according to a
second embodiment.
[0012] FIG. 9 is a side view of a fixing apparatus according to a
third embodiment.
DETAILED DESCRIPTION
[0013] In accordance with an embodiment, a fixing apparatus
includes a belt including a ferromagnetic layer, a roller facing
the belt, a ferromagnetic plate disposed inside the belt and having
a Curie point that is lower than a Curie point of the ferromagnetic
layer, an induction heater configured to cause heat generation in
the magnetic layer and the ferromagnetic plate, a driving circuit
configured to output a high frequency current to the induction
heater by switching on and off switching elements thereof, and a
controller. An image on a sheet is fixed as the sheet passes
through a nip formed between the belt and the roller. The
controller is configured to determine whether or not a temperature
of the ferromagnetic plate is higher than the Curie point thereof,
and control the driving circuit to reduce a level of the high
frequency current when the temperature of the ferromagnetic plate
is determined to be higher than the Curie point thereof.
First Embodiment
[0014] Hereinafter an image forming apparatus 10 according to a
first embodiment is described with reference to the accompanying
drawings. Further, the same components are applied with same
reference numerals in the drawings.
[0015] FIG. 1 is a side view of the image forming apparatus 10
according to the first embodiment. A Multi-function peripheral
(MFP) is described below as an example of the image forming
apparatus 10.
[0016] As shown in FIG. 1, the MFP 10 includes a scanner 12, a
control panel 13, and a main body section 14. Each of the scanner
12, the control panel 13, and the main body section 14 includes a
control section. The MFP 10 includes a system control unit 100
serving as the control section for the scanner 12, the control
panel 13 and the main body section 14.
[0017] The system control unit 100 includes a CPU (Central
Processing Unit) 100a, an ROM (Read Only Memory) 100b, and an RAM
(Random Access Memory) 100c (refer to FIG. 5). The system control
unit 100 controls a main control circuit 101 (refer to FIG. 2)
serving as the control section for the main body section 14.
[0018] The main control circuit 101 includes a CPU, a ROM and a RAM
(none are shown). The main body section 14 includes a paper feed
cassette unit 16, a printer unit 18, a fixing apparatus 34, and the
like. The main control circuit 101 controls the paper feed cassette
unit 16, the printer unit 18, the fixing apparatus 34, and the
like.
[0019] The scanner 12 reads an image of a document. The control
panel 13 has input keys 13a and a display unit 13b. For example,
the input keys 13a receive an input from a user. For example, the
display unit 13b is of a touch panel type. The display unit 13b
receives an input from the user and displays information to the
user.
[0020] The paper feed cassette unit 16 includes a paper feed
cassette 16a and a pickup roller 16b. The paper feed cassette 16a
stores sheets P serving as media. The pickup roller 16b picks up
the sheet P from the paper feed cassette 16a. The paper feed
cassette 16a is provided to store sheets P. A paper feed tray 17 is
provided to feed sheets P with a pickup roller 17a.
[0021] The printer unit 18 forms an image. For example, the printer
unit 18 carries out image forming processing of the image of the
document read by the scanner 12. The printer unit 18 includes an
intermediate transfer belt 21. In the printer unit 18, the
intermediate transfer belt 21 is supported by a backup roller 40, a
driven roller 41, and a tension roller 42. The backup roller 40
includes a driving unit (not shown) and configured to rotate on its
own. In the printer unit 18, the intermediate transfer belt 21
rotates in a direction indicated by an arrow m.
[0022] The printer unit 18 further includes four sets of image
forming stations 22Y, 22M, 22C, and 22K. The image forming stations
22Y, 22M, 22C, and 22K are used to respectively form a yellow (Y)
image, a magenta (M) image, a cyan (C) image, and a black (K)
image. The image forming stations 22Y, 22M, 22C, and 22K are
arranged in parallel to each other along the rotational direction
of the intermediate transfer belt 21 below the intermediate
transfer belt 21.
[0023] The printer unit 18 further includes cartridges 23Y, 23M,
23C, and 23K above the image forming stations 22Y, 22M, 22C, and
22K, respectively. The cartridges 23Y, 23M, 23C, and 23K store
toner for replenishment of yellow (Y), magenta (M), cyan (C), and
black (K), respectively.
[0024] Hereinafter, the image forming station 22Y for forming a
yellow (Y) image among the image forming stations 22Y, 22M, 22C,
and 22K is described as an example. Further, as the configurations
of the image forming stations 22M, 22C, and 22K are the same as the
configuration of the image forming station 22Y, the detailed
description thereof is not provided.
[0025] The image forming station 22Y includes a charger 26, an
exposure scanning head 27, a developing device 28, and a
photoconductive cleaner 29. The charger 26, the exposure scanning
head 27, the developing device 28, and the photoconductive cleaner
29 are arranged around a photoconductive drum 24 that is configured
to rotate in a direction indicated by an arrow n.
[0026] The image forming station 22Y includes a primary transfer
roller 30. The primary transfer roller 30 faces the photoconductive
drum 24 across the intermediate transfer belt 21.
[0027] The image forming station 22Y exposes the photoconductive
drum 24 that is charged by the charger 26 through the exposure
scanning head 27. The image forming station 22Y forms an
electrostatic latent image on the photoconductive drum 24. The
developing device 28 develops the electrostatic latent image on the
photoconductive drum 24 using a two-component developing agent
including toner and carrier.
[0028] The primary transfer roller 30 primarily transfers the toner
image formed on the photoconductive drum 24 to the intermediate
transfer belt 21. The image forming stations 22Y, 22M, 22C, and 22K
form a color (full-color) toner image on the intermediate transfer
belt 21 through the primary transfer rollers 30. The color toner
image is formed by overlapping toner images of Y (yellow), M
(magenta), C (cyan), and K (black) in sequence. The photoconductive
cleaner 29 removes the toner left on the photoconductive drum 24
after the primary transfer.
[0029] The printer unit 18 further includes a secondary transfer
roller 32. The secondary transfer roller 32 faces the backup roller
40 across the intermediate transfer belt 21. The secondary transfer
roller 32 operates to transfer the color toner image on the
intermediate transfer belt 21 to the sheet P. The sheet P is fed by
the paper feed cassette unit 16 or the manual paper feeding tray 17
along the conveyance path 33.
[0030] The printer unit 18 further includes a belt cleaner 43
facing the driven roller 41 across the intermediate transfer belt
21. The belt cleaner 43 removes toner left on the intermediate
transfer belt 21 after the secondary transfer.
[0031] The printer unit 18 also includes a register roller 33a, a
fixing apparatus 34, and a sheet discharge roller 36, along the
conveyance path 33. The printer unit 18 further includes a
bifurcating unit 37 and a reversal conveyance unit 38 at the
downstream side of the fixing apparatus 34 in a sheet conveying
direction. The bifurcating unit 37 sends the sheet P subjected to
fixing processing to a sheet discharge unit 20 or the reversal
conveyance unit 38. In a case of duplex printing, the reversal
conveyance unit 38 reverses the sheet P sent from the bifurcating
unit 37 to a direction of the register roller 33a and conveys the
sheet P. The MFP 10 forms a fixed toner image on the sheet P with
the printer unit 18 and then discharges it to the sheet discharge
unit 20.
[0032] Further, the MFP 10 is not limited to the tandem developing
system, and the number of the developing devices 28 is also not
limited. Further, the MFP 10 may transfer the toner image from the
photoconductive drum 24 to the sheet P directly.
[0033] Hereinafter, the fixing apparatus 34 is described in
detail.
[0034] FIG. 2 is a side view of the fixing apparatus 34, including
an electromagnetic induction heating coil unit 52 (induction
current generation section), and illustrates a control block of a
main control circuit 101 (control section) according to the first
embodiment. Hereinafter, the electromagnetic induction heating coil
unit is referred to as an "IH coil unit".
[0035] As shown in FIG. 2, the fixing apparatus 34 includes a
fixing belt 50, a press roller 51, an IH coil unit 52, an auxiliary
heating plate 69 (auxiliary heating section), a second coil unit 84
(measurement section), and the main control circuit 101.
[0036] The fixing belt 50 is a cylindrical endless belt. In the
inner peripheral space of the fixing belt 50, an internal belt
mechanism 55 including a nip pad 53 and the auxiliary heating plate
69 is arranged.
[0037] The fixing belt 50 is formed by laminating a heating layer
50a (conductive layer) serving as a heating unit and a releasing
layer 50c in sequence on a base layer 50b (refer to FIG. 4).
Further, as long as the fixing belt 50 includes the heating layer
50a, no limitation is given to the layer structure.
[0038] The base layer 50b is made from, for example, polyimide
resin (PI). The heating layer 50a is formed of, for example, a
non-magnetic metal such as copper (Cu). The releasing layer 50c is
made from, for example, fluorine resin such as PFA
(Tetrafluoroethylene Perfluoro alkyl vinyl ether copolymer
resin).
[0039] In order to rapidly warm up the fixing belt 50, the heating
layer 50a is thin and the heat capacity of the fixing belt 50 is
low. The fixing belt 50 having low heat capacity shortens the time
required to warm up the fixing belt 50 and saves energy consumption
for the warming-up.
[0040] In order to reduce the heat capacity of the fixing belt 50,
the thickness of the copper layer of the heating layer 50a is, for
example, 10 .mu.m. Further, the heating layer 50a is coated by a
protective layer such as nickel. The nickel protective layer can
suppress the oxidation of copper layer, which can improve the
mechanical strength of the fixing belt 50.
[0041] Further, the heating layer 50a may be formed by performing
copper plating as well as electroless nickel plating on the base
layer 50b made from polyimide resin. By performing the electroless
nickel plating, the adhesion strength of the base layer 50b to the
heating layer 50a is improved, and the mechanical strength of the
fixing belt 50 is also improved.
[0042] Further, the surface of the base layer 50b may be roughened
through sandblast or chemical etching. By roughening the surface of
the base layer 50b, the adhesion strength of the base layer 50b to
the nickel plating of the heating layer 50a is further
improved.
[0043] Further, a metal such as titanium (Ti) may be dispersed into
the polyimide resin for forming the base layer 50b. By dispersing
metal into the base layer 50b, the adhesion strength of the base
layer 50b to the nickel plating of the heating layer 50a is further
improved.
[0044] For example, the heating layer 50a may be made from nickel,
iron (Fe), stainless steel, aluminum (Al), silver (Ag), or the
like. The heating layer 50a may use two or more kinds of alloys,
and may also be formed by stacking two or more kinds of metal in
layer.
[0045] FIG. 3 is a perspective view of the IH coil unit 52
according to the first embodiment.
[0046] As shown in FIG. 3, the IH coil unit 52 includes a main coil
56 (first coil), a first core 57, and a second core 58.
[0047] The main coil 56 generates magnetic flux in accordance with
application of a high frequency current. The main coil 56 is
arranged at the outer peripheral side of the fixing belt 50. The
main coil 56 faces the fixing belt 50 in the thickness direction.
The longitudinal direction of the main coil 56 is parallel to the
width direction of the fixing belt 50 (hereinafter referred to as a
"belt width direction").
[0048] The first core 57 and the second core 58 cover a side of the
main coil 56 (hereinafter referred to as "backside") that is
opposite to the side that faces the fixing belt 50. The first core
57 and the second core 58 prevent leakage of the magnetic flux
generated by the main coil 56 at the back side. The first core 57
and the second core 58 concentrate the magnetic flux from the main
coil 56 on the fixing belt 50.
[0049] The first core 57 includes a plurality of single wing parts
57a. The plurality of single wing parts 57a is alternately arranged
in a staggered manner with a center line 56d along the longitudinal
direction of the main coil 56 as an axis of symmetry.
[0050] The second core 58 is arranged at both sides of the first
core 57in the longitudinal direction thereof. The second core 58
includes a plurality of dual wing parts 58a across both sides of
the main coil 56. For example, the single wing part 57a and the
dual wing part 58a may be made from magnetic material such as a
nickel-zinc alloy (Ni--Zn), a manganese-nickel alloy (Mn--Ni), and
the like.
[0051] The first core 57 regulates the magnetic flux generated by
the main coil 56 with the plurality of the single wing parts 57a.
The magnetic flux generated by the main coil 56 may be regulated
alternately by every single wing part 57a with a central line 56d
as an axis of symmetry. The first core 57 concentrates the magnetic
flux from the main coil 56 on the fixing belt 50 with the plurality
of single wing parts 57a.
[0052] The second core 58 regulates the magnetic flux generated by
the main coil 56 with the plurality of dual wing parts 58a. The
magnetic flux generated by the main coil 56 is regulated by the
dual wing parts 58a at both sides of the first core 57. The second
core 58 concentrates the magnetic flux from the main coil 56 on the
fixing belt 50 with the plurality of dual wing parts 58a. The
magnetic flux concentration caused by the second core 58 is greater
than the magnetic flux concentration caused by the first core
57.
[0053] The main coil 56 includes first wings 56a and second wings
56b. The first wings 56a are arranged at one side of the central
line 56d. The second wings 56b are arranged at the other side of
the central line 56d. Window parts 56c are formed between the first
wings 56a and the second wings 56b at the inner side in the
longitudinal direction of the main coil 56.
[0054] For example, the main coil 56 uses litz wire. The litz wire
is formed of a plurality of bundles of copper wire material that is
coated by the heat-resistant polyamide-imide serving as an
insulated material. The main coil 56 is formed by winding the
conductive coils.
[0055] As shown in FIG. 2, the main coil 56 generates the magnetic
flux through the application of the high frequency current from the
inverter driving circuit 68. For example, the inverter driving
circuit 68 includes switching elements including the IGBT
(Insulated Gate Bipolar Transistor) element 68a, a MOSFET (Metal
Oxide semiconductor field effect Transistor) element (not shown),
and the like. The IGBT element 68a is connected to the MOSFET
element. By alternately turning on/off the IGBT element 68a and the
MOSFET element, a high frequency current flows into the main coil
56. By the flow of the high frequency current into the main coil
56, a high frequency magnetic field is generated around the main
coil 56. Through the magnetic flux of the high frequency magnetic
field, an eddy current is generated in the heating layer 50a of the
fixing belt 50. Further, Joule heat is generated due to the eddy
current flowing in the heating layer 50a that has electric
resistance. As a result, the fixing belt 50 is heated.
[0056] For example, it is assumed that the "ON" period of the IGBT
element 68a is constant. By varying the "ON" period of the MOSFET
element, the high frequency current flowing into the main coil 56
changes. With the change of high frequency current flowing into the
main coil 56, the output of the electromagnetic induction heating
also changes.
[0057] The auxiliary heating plate 69 is arranged at the inner
peripheral side of the fixing belt 50. Seen in a belt width
direction, the auxiliary heating plate 69 has an arc shape and is
arranged along the inner peripheral surface of the fixing belt 50.
The auxiliary heating plate 69 faces the main coil 56 across the
fixing belt 50. The auxiliary heating plate 69 is formed of a
magnetic material (ferromagnetic material) of which the Curie point
is lower than that of the heating layer 50a. Magnetic flux through
the auxiliary heating plate 69 and the fixing belt 50 is generated
by the magnetic flux generated by the main coil 56. Consequently,
Joule heat is generated in the heating layer 50a. The generated
Joule heat is used to further heat the fixing belt 50 by the main
coil 56.
[0058] The auxiliary heating plate 69 is supported by sills (not
shown) at the arc-shaped both ends thereof. The outer surface of
the auxiliary heating plate 69 in the diameter direction (radial
direction) is separated from the inner peripheral surface of the
fixing belt 50. For example, the length of a gap between the outer
surface of the auxiliary heating plate 69 and the inner peripheral
surface of the fixing belt 50 is about 1.about.2 mm. Alternatively,
the outer surface of the auxiliary heating plate 69 may be in
contact with the inner peripheral surface of the fixing belt
50.
[0059] For example, the auxiliary heating plate 69 in the belt
width direction is longer than a sheet-passing area in the belt
width direction (hereinafter referred to as a "sheet width"). In
addition, the sheet width is a width of a sheet of which the
short-side width is longest among sheets that can be used in the
MFP 10. For example, it is assumed that the sheet width is a little
longer than the short-side width of A3-sized paper.
[0060] FIG. 4 illustrates magnetic paths of the magnetic flux
generated by the main coil 56 according to the first embodiment.
The magnetic paths extend through the fixing belt 50 and/or the
auxiliary heating plate 69.
[0061] As shown in FIG. 4, the magnetic flux generated by the main
coil 56 forms a first magnetic path 81 extending through the
heating layer 50a of the fixing belt 50. The first magnetic path 81
is formed in such a manner that the first wings 56a and the second
wings 56b of the main coil 56 are surrounded. The first magnetic
path 81 passes through the first core 57, the second core 58, and
the heating layer 50a. Further, the magnetic flux generated by the
main coil 56 forms a second magnetic path 82 that extends through
the auxiliary heating plate 69. The second magnetic path 82 is
formed at a position adjacent to the first magnetic path 81 in a
diameter direction of the fixing belt 50 (hereinafter referred to
as a "belt diameter direction"). The second magnetic path 82 passes
through the auxiliary heating plate 69 and the heating layer
50a.
[0062] The auxiliary heating plate 69 is formed of thin metal
member made from the magnetic shunt alloy such as iron or nickel
alloy of which the Curie point is 220.about.230 degrees centigrade.
When the temperature of the auxiliary heating plate 69 exceeds the
Curie point thereof, the auxiliary heating plate 69 will lose its
magnetism. Specifically, when the Curie point is exceeded, the
magnetism of the auxiliary heating plate 69 is changed from
ferromagnetism to paramagnetism. When the temperature of the
auxiliary heating plate 69 exceeds the Curie point thereof, the
second magnetic path 82 is not formed and consequently there is no
assistance to the heat of the fixing belt 50. By forming the
auxiliary heating plate 69 with the magnetic shunt alloy, the
auxiliary heating plate 69 can be used to assist the rise of
temperature of the fixing belt 50 when the temperature is lower
than the Curie point thereof, and can be used to prevent the
excessive temperature rise of the fixing belt 50 when the
temperature is higher than the Curie point thereof.
[0063] Herein, the heating of the fixing belt 50 is adjusted
through electric control of an IH control circuit 67. To keep the
belt temperature constant, the output of the IH coil unit 52 is
controlled to be constant. If the auxiliary heating plate 69 is
formed of magnetic shunt alloy, the auxiliary heating plate 69
loses its magnetism when the temperature thereof exceeds the Curie
point, and the second magnetic path 82 is not formed. Consequently,
the load (electric resistance) of the IH coil unit 52
decreases.
[0064] The IH control circuit 67 increases the current flowing
through the inverter driving circuit 68 corresponding to the
reduction of load of the IH coil unit 52 so as to keep the output
of the IH coil unit 52 constant. If the current flowing through the
inverter driving circuit 68 is increased, the current flowing in
the IGBT element 68a is also increased. If so, the temperature of
the IGBT element 68a rises excessively, and the IGBT element 68a
may be damaged. To prevent this, in the present embodiment, the
change of magnetism of the auxiliary heating plate 69 is estimated
by measuring the electric resistance of the second coil 84a. In
addition, the IH control circuit 67 controls the IH coil unit 52 to
reduce the output of the electromagnetic induction heating when the
measured electric resistance is smaller than a threshold value.
[0065] Further, the auxiliary heating plate 69 may be formed of a
thin metal member having magnetic characteristic such as iron,
nickel, stainless steel, and the like. As long as the material has
magnetic characteristic, the auxiliary heating plate 69 may be
formed of resin including magnetic powder. Alternatively, the
auxiliary heating plate 69 may be formed of the magnetic material,
ferrite. The magnetic material, ferrite, promotes the heating of
the fixing belt 50 through the magnetic flux generated by the
induction current. The magnetic material, ferrite, itself does not
generate heat even if the magnetic flux generated by induction
current passes through it. The auxiliary heating plate 69 is not
limited to a thin plate member.
[0066] As shown in FIG. 2, a shield 76 is arranged on the inner
peripheral side of the auxiliary heating plate 69. The shield 76
has an arc shape similar to the shape of the auxiliary heating
plate 69. The shield 76 is supported by sills (not shown) at the
arc-shaped ends thereof. Further, the shield 76 may support the
auxiliary heating plate 69. For example, the shield 76 is made from
non-magnetic material such as aluminum, copper, and the like. The
shield 76 shields the magnetic flux from the IH coil unit 52. The
shield 76 suppresses an influence on a voltage and the like
measured by a thermistor by the magnetic flux.
[0067] A nip pad 53 is a pressing unit positioned to press the
inner peripheral surface of the fixing belt 50 against the side of
the press roller 51. A nip 54 is formed between the fixing belt 50
and the press roller 51. The nip pad 53 has a nip forming surface
53a, and the nip 54 is formed between the fixing belt 50 pressed by
the nip pad 53 and the press roller 51. Seen in the belt width
direction, the nip forming surface 53a is curved towards the inner
peripheral side of the fixing belt 50 along the outer peripheral
surface of the press roller 51.
[0068] For example, the press roller 51 includes a heat-resistant
silicon sponge layer and a silicon rubber layer around the core
bar. For example, a releasing layer is arranged on the surface of
the press roller 51. The releasing layer is made from fluorine
resin such as the PFA resin and the like. The press roller 51
presses the fixing belt 50 through a pressing mechanism. The press
roller 51 and the nip pad 53 serve as a pressing unit which presses
the fixing belt 50.
[0069] A motor 51b is arranged as a driving unit of the fixing belt
50 and the press roller 51. The motor 51b is energized by a motor
driving circuit 51c that is controlled by the main control circuit
101. The motor 51b is connected to the press roller 51 through a
first gear train (not shown). The motor 51b is connected to a belt
driving member through a second gear train and a one-way clutch
(none is shown). The motor 51b causes the press roller 51 to rotate
in a direction indicated by an arrow q. When the fixing belt 50 is
in contact with the press roller 51, the fixing belt 50 is rotated
by the press roller 51 in a direction indicated by an arrow u. When
the fixing belt 50 is separated from the press roller 51, the motor
51b directly controls the fixing belt 50 to rotate in the direction
indicated by the arrow u. The fixing belt 50 may include a driving
unit separately from the driving unit of the press roller 51.
[0070] A center thermistor 61 and an edge thermistor 62 measure a
belt temperature. The measured results of the belt temperature are
input to the main control circuit 101. The center thermistor 61 is
arranged at a center of the IH coil unit 52 in the belt width
direction. The edge thermistor 62 is arranged in a non-paper
passing, heating area of the IH coil unit 52 in the belt width
direction. The main control circuit 101 controls the IH coil unit
52 in such a manner that the electromagnetic induction heating is
stopped when the belt temperature measured by the edge thermistor
62 is greater than a threshold value. The electromagnetic induction
heating is stopped when the temperature of the non-paper passing,
heating area of the fixing belt 50 rises excessively, and thereby
preventing the fixing belt 50 from being damaged.
[0071] The main control circuit 101 controls the IH control circuit
67 according to the measured results of the belt temperature by the
center thermistor 61 and the edge thermistor 62. The IH control
circuit 67 controls the magnitude of the high frequency current
output by the inverter driving circuit 68 under the control of the
main control circuit 101. The fixing belt 50 is maintained within
control temperature ranges in accordance with the output of the
inverter driving circuit 68. The IH control circuit 67 includes a
CPU, a ROM, and a RAM (none are shown).
[0072] A thermostat 63 functions as a safety device of the fixing
apparatus 34. The thermostat 63 is operated when the fixing belt 50
generates heat abnormally and the temperature thereof rises to its
shut-off threshold value. With the operation of the thermostat 63,
the current flowing to the IH coil unit 52 is shut off. In this
way, the MFP 10 stops driving, and thereby preventing the fixing
apparatus 34 from being abnormally heated.
[0073] Hereinafter, a control system 110 of the IH coil unit 52
which enables the fixing belt 50 to generate heat is described in
detail.
[0074] FIG. 5 is a block diagram of the control system 110 which
mainly controls the IH coil unit 52 according to the first
embodiment.
[0075] As shown in FIG. 5, the control system 110 includes a system
control unit 100, the main control circuit 101, an IH circuit 120,
and the motor driving circuit 51c.
[0076] The control system 110 supplies power to the IH coil unit 52
through the IH circuit 120. The IH circuit 120 includes a rectifier
circuit 121, the IH control circuit 67, the inverter driving
circuit 68, and a current measurement circuit 122.
[0077] Current is supplied from an AC power supply 111 to the IH
circuit 120 through a relay 112. The IH circuit 120 rectifies the
current with the rectifier circuit 121 and then supplies the
current to the inverter driving circuit 68. When the thermostat 63
is cut off, the relay 112 shuts off the current from the AC power
supply 111. The inverter driving circuit 68 includes a drive IC 68b
of the IGBT element 68a. The IH control circuit 67 controls the
drive IC 68b according to the measured results of the belt
temperature by the center thermistor 61 and the edge thermistor 62.
The IH control circuit 67 controls the drive IC 68b to control the
output of the IGBT element 68a. The current measurement circuit 122
sends the measured results output by the IGBT element 68a to the IH
control circuit 67. The IH control circuit 67 controls the drive IC
68b based on the measured results output from the current
measurement circuit 122, so that the output of the IH coil unit 52
is constant.
[0078] The main control circuit 101 acquires a measurement value R
(refer to FIG. 7) from an electric resistance measurement circuit
84b. The main control circuit 101 controls the IH coil unit 52
based on the measurement value R. Specifically, the main control
circuit 101 determines whether or not the measurement value R is
smaller than a threshold value Rt. Then, the main control circuit
101 controls either the continuous driving of the fixing apparatus
34 and the reduction of the output of the IH coil unit 52 based on
the determination results. Here, the reduction of output of the IH
coil unit 52 includes the stopping of the IH coil unit 52.
[0079] FIG. 6 is a side view of the main portions of the fixing
apparatus 34 according to the first embodiment.
[0080] As shown in FIG. 6, the second coil unit 84 includes a
second coil 84a and the electric resistance measurement circuit 84b
(electric resistance measurement section). The second coil unit 84
measures whether or not the temperature of the auxiliary heating
plate 69 exceeds the Curie point thereof. The second coil 84a is
arranged separately from the main coil 56. The second coil 84a
generates a magnetic field passing through the auxiliary heating
plate 69 through energization. For example, the second coil 84a
includes winding wire of litz wire. The electric resistance
measurement circuit 84b measures the electric resistance of the
second coil 84a. The measured result of the electric resistance of
the second coil 84a is input to the main control circuit 101.
[0081] Hereinafter, it is assumed that an area of the auxiliary
heating plate 69 that faces the IH coil unit 52 across the fixing
belt 50 and extends along a circumferential direction of the fixing
belt 50 (hereinafter referred to as a "belt circumferential
direction") is a facing area 69a. Further, it is assumed that an
end portion 69c of the auxiliary heating plate 69 is an end portion
of the auxiliary heating plate 69 in the belt circumferential
direction, and is an area adjacent to the facing area 69a. The end
portion 69c of the auxiliary heating plate 69 does not face the IH
coil unit 52 across the fixing belt 50 in the belt diameter
direction.
[0082] Further, an end portion 52c of the IH coil unit 52 is an end
portion in the belt circumferential direction of each of the first
core 57 and the second core 58, and includes a portion protruding
to the inner side in the belt diameter direction.
[0083] The second coil 84a is arranged in an area S1 (refer to FIG.
2) which faces the auxiliary heating plate 69 but does not face the
main coil 56. Specifically, the area S1 is positioned between the
end portion 52c of the IH coil unit 52 and the fixing belt 50 in
the belt diameter direction. The area S1 ranges from the outer side
of the main coil 56 to the end portion 69c of the auxiliary heating
plate 69 in the belt circumferential direction. The area S1 faces
not only the end portion 52c of the IH coil unit 52 but also the
end portion 69c of the auxiliary heating plate 69 across the fixing
belt 50 in the belt circumferential direction. One end (end at the
inner side) of the area S1 in the belt circumferential direction
faces a boundary between the end portion 52c of the IH coil unit 52
and the main coil 56 in the belt diameter direction. The other end
(an end at the outer side) of the area S1 in the belt width
direction faces front ends (arc-shaped both ends) of the end
portion 69c of the auxiliary heating plate 69 across the fixing
belt 50 in the belt diameter direction.
[0084] In the present embodiment, the second coil 84a is arranged
at the outer peripheral side of the fixing belt 50. The second coil
84a faces the end portion 69c of the auxiliary heating plate 69
across the fixing belt 50.
[0085] Further, the second coil 84a may face the facing area 69a of
the auxiliary heating plate 69 across the fixing belt 50 in a range
where the second coil 84a does not face the main coil 56.
[0086] The second coil 84a is fixed separately from the fixing belt
50 at a given interval. The second coil 84a at least faces the
paper passing area in the belt width direction. For example, the
second coil 84a faces a central portion of the fixing belt 50.
[0087] The size of the second coil 84a is smaller than that of the
main coil 56, because the second coil 84a is used to generate a
magnetic field passing through the auxiliary heating plate 69 that
is sufficient for the electric resistance measurement circuit 84b
to measure the electric resistance of the second coil 84a.
[0088] When compared to a case in which the size of the second coil
84a is identical to or larger than the size of the main coil 56, it
is possible to arrange the second coil 84a in the area S1
easier.
[0089] The magnetic flux generated by the second coil 84a forms a
third magnetic path 85 that extends through the heating layer 50a
of the fixing belt 50. Further, the magnetic flux generated by the
second coil 84a forms a fourth magnetic path 86 that extends
through the auxiliary heating plate 69 before auxiliary heating
plate 69 loses its magnetism due to the temperature thereof
exceeding the Curie point thereof. The fourth magnetic path 86 is
formed at a position adjacent to the third magnetic path 85 in the
belt diameter direction. The fourth magnetic path 86 passes through
the auxiliary heating plate 69 and the heating layer 50a. The
electric resistance of the second coil 84a varies in accordance
with change of the magnetism of the auxiliary heating plate 69.
That is, the electric resistance of the second coil 84a varies
according to whether or not the fourth magnetic path 86 is
formed.
[0090] A weak high frequency current (hereinafter referred to as a
"high frequency weak current") flows into the second coil 84a, and
this current enables the electric resistance measurement circuit
84b to measure the electric resistance of the second coil 84a. For
example, the electric resistance measurement circuit 84b is
connected at an upstream side and a downstream side of the second
coil 84a in a current flowing direction, and the aforementioned
electric resistance is measured according to the values of current
respectively at the upstream side and the downstream side of the
second coil 84a. Further, it is assumed that the high frequency
weak current is weaker than the high frequency current output from
the inverter driving circuit 68.
[0091] Next, an example of an operation of the fixing apparatus 34
according to the first embodiment is described with reference to
FIG. 7.
[0092] FIG. 7 is a flowchart illustrating an operation of the
fixing apparatus 34 according to the first embodiment.
[0093] In ACT 100, the electric resistance measurement circuit 84b
causes the high frequency weak current to flow into the second coil
84a. It is assumed that, for example, the frequency and current of
the high frequency weak current are respectively 60 kHz and 10
mA.
[0094] In ACT 101, the electric resistance measurement circuit 84b
measures the electric resistance of the second coil 84a. It is
assumed in the present embodiment that the electric resistance of
the second coil 84a measured by the electric resistance measurement
circuit 84b is a "measured value R". The main control circuit 101
acquires the measured value R from the electric resistance
measurement circuit 84b.
[0095] Alternatively, the main control circuit 101 may acquire the
measured value R from other circuit such as a logic circuit.
[0096] In ACT 102, the main control circuit 101 determines whether
or not the measured value R acquired in ACT 101 is smaller than a
threshold value Rt (for example, 1".OMEGA.").
[0097] By determining whether or not the measured value R is
smaller than the threshold value Rt, it is possible to determine
the change of magnetism of the auxiliary heating plate 69 for the
following reasons.
[0098] When the measured value R is greater than the threshold
value Rt, the auxiliary heating plate 69 has ferromagnetism because
its temperature is lower than the Curie point thereof. When the
auxiliary heating plate 69 has ferromagnetism, the magnetic flux
generated by the second coil 84a forms the third magnetic path 85
and the fourth magnetic path 86.
[0099] On the other hand, when the measured value R is smaller than
the threshold value Rt, the auxiliary heating plate 69 has
paramagnetism because its temperature is higher than the Curie
point thereof. In such a case, the fourth magnetic path 86 is not
formed.
[0100] Thus, it is possible to estimate the magnetism of the
auxiliary heating plate 69 by determining whether or not the
measured value R is smaller than the threshold value Rt.
[0101] When the main control circuit 101 determines that the
measured value R is smaller than the threshold value Rt (YES in ACT
102), the process proceeds to ACT 104. When the main control
circuit 101 determines that the measured value R is greater than
the threshold value Rt (NO in ACT 102), the process proceeds to ACT
103.
[0102] In ACT 103, the fixing apparatus 34 continues its driving.
For example, when performing a high output driving such as a
continuous paper passing and the warming up, the fixing apparatus
34 continues the high output driving.
[0103] In ACT 104, the main control circuit 101 controls the IH
coil unit 52 based on the measured value R.
[0104] In ACT 105, the main control circuit 101 determines whether
to stop the IH coil unit 52, based on the measured value R. For
example, when the measured value R is smaller than 0.5".OMEGA.",
the main control circuit 101 determines to stop the IH coil unit
52. If it is determined to stop the IH coil unit 52 (YES in ACT
105), the main control circuit 101 terminates the processing. By
stopping the IH coil unit 52, the main control circuit 101
suppresses the excessive temperature rise of the IGBT element 68a.
Consequently, the main control circuit 101 prevents the IGBT
element 68a from being damaged.
[0105] If it is determined not to stop the IH coil unit 52 (NO in
ACT 105), the process proceed to ACT 106.
[0106] In ACT 106, the main control circuit 101 reduces the output
of the IH coil unit 52. For example, the main control circuit 101
reduces the power supplied to the IH coil unit 52. By reducing the
output of the IH coil unit 52, the main control circuit 101
suppresses the excessive temperature rise of the IGBT element 68a.
Consequently, the main control circuit 101 prevents the IGBT
element 68a from being damaged.
[0107] In ACT 103, the fixing apparatus 34 continues its driving
when the output of the IH coil unit 52 is reduced.
[0108] Hereinafter, the operation of the fixing apparatus 34 during
a warming-up operation is described.
[0109] As shown in FIG. 2, the fixing apparatus 34 rotates the
fixing belt 50 in the direction indicated by the arrow u during the
warming-up operation. By applying the high frequency current
through the inverter driving circuit 68, the IH coil unit 52
generates magnetic flux at the side of the fixing belt 50.
[0110] For example, the fixing belt 50 is rotated in the direction
indicated by the arrow u when the fixing belt 50 is separated from
the press roller 51 during the warming-up operation. Compared to a
case of rotating the fixing belt 50 when the fixing belt 50 is in
contact with the press roller 51, it is possible to avoid the heat
of the fixing belt 50 from transferring to the press roller 51.
Consequently, it is possible to shorten the period of the warming
up operation.
[0111] Alternatively, the fixing belt 50 may be driven to rotate in
the direction indicated by the arrow u by rotating the press roller
51 in the direction indicated by the arrow q when the press roller
51 is in contact with the fixing belt 50 at the time of warming
up.
[0112] As shown in FIG. 4, the IH coil unit 52 heats the fixing
belt 50 through the first magnetic path 81. The auxiliary heating
plate 69 assists the IH coil unit 52 in heating the fixing belt 50
through the second magnetic path 82. In this way, it enables the
rapid warming up of the fixing belt 50.
[0113] As shown in FIG. 2, the IH control circuit 67 controls the
inverter driving circuit 68 according to the measured results of
the belt temperatures measured by the center thermistor 61 and the
edge thermistor 62. The inverter driving circuit 68 supplies the
high frequency current to the main coil 56.
[0114] Hereinafter, the operations of the fixing apparatus 34
during a fixing operation are described.
[0115] After the fixing belt 50 reaches the fixing temperature and
the warming-up operation is ended, the press roller 51 is moved to
be in contact with the fixing belt 50. In a state in which the
press roller 51 is in contact with the fixing belt 50, the fixing
belt 50 may be driven to rotate in the direction indicated by the
arrow u by rotating the press roller 51 in the direction indicated
by the arrow q. When a print request is received, the MFP 10 (refer
to FIG. 1) starts a print operation in response. The MFP 10 forms a
toner image on the sheet P with the printer unit 18, and conveys
the sheet P to the fixing apparatus 34.
[0116] The MFP 10 causes the sheet P on which the toner image is
formed to pass through the nip 54 between the fixing belt 50 that
reaches the fixing temperature and the press roller 51. The fixing
apparatus 34 fixes the toner image on the sheet P. During the
fixing operation, the IH control circuit 67 controls the IH coil
unit 52 to keep the fixing temperature of the fixing belt 50
constant.
[0117] Through the fixing operation, the heat of the fixing belt 50
transfers to the sheet P. For example, when a plurality of sheets P
continuously passes at a high speed, since the amount of heat
transferring to the sheets P is large, the fixing belt 50 having
low heat capacity may not be maintained at the fixing temperature.
The heat caused by the second magnetic path 82 supplements heating
of the fixing belt 50. As a result, the belt temperature can be
maintained at the fixing temperature even in the continuous paper
passing at a high speed.
[0118] Here, disposing a thermistor which measures the temperature
of the IGBT element 68a would be useful to prevent the IGBT element
68a from being damaged. In such a case, the thermistor would be
installed in a case of the inverter driving circuit 68 but not in
the IGBT element 68a itself. When the thermistor measures a
temperature rise of the IGBT element 68a, the main control circuit
101 would drive a fan to cool the IGBT element 68a. Through the
thermistor, the gentle temperature rise of the IGBT element 68a may
be measured. However, it is difficult for the thermistor to measure
a sudden temperature rise. Further, as the thermistor is installed
in the case, it is difficult for the thermistor to measure the
accurate temperature of the IGBT element 68a. The measured
temperature of the IGBT element 68a by the thermistor may be
divergent from the actual temperature of the IGBT element 68a.
Moreover, the cooling of the IGBT element 68a by the fan may not
sufficiently cool the internal portion of the IGBT element 68a.
Thus, the damage of the IGBT element 68a cannot be sufficiently
prevented by the temperature measurement of the thermistor and the
cooling process by the fan.
[0119] To the contrary, according to the first embodiment, the
electric resistance measurement circuit 84b measures the electric
resistance of the second coil 84a. By measuring the electric
resistance of the second coil 84a, it is possible to measure not
only the gentle temperature rise but also the sudden temperature
rise of the IGBT element 68a indirectly. Compared with the case of
arranging the thermistor described above, it is possible to measure
the temperature of the IGBT element 68a in real time by measuring
the electric resistance of the second coil 84a. Furthermore,
difference between the measured temperature and the actual
temperature of the IGBT element 68a would not be an issue in the
present embodiment.
[0120] Further, the main control circuit 101 acquires the electric
resistance (measured value R) of the second coil 84a from the
electric resistance measurement circuit 84b. The main control
circuit 101 controls the IH coil unit 52 to weaken the
electromagnetic induction heating when the measured value R is
smaller than a threshold value. By weakening the electromagnetic
induction heating when the measured value R is smaller than the
threshold value, it is possible to suppress the excessive
temperature rise of the IGBT element 68a. Specifically, the main
control circuit 101 determines whether or not the measured value R
is smaller than the threshold value Rt. If it is determined that
the measured value R is smaller than the threshold value Rt, the
main control circuit 101 reduces heat generation caused by the IH
coil unit 52. For example, it is possible to suppress the excessive
temperature rise of the IGBT element 68a by stopping the IH coil
unit 52 or by reducing the heat generation by the IH coil unit 52.
As a result, it is possible to prevent the IGBT element 68a from
being damaged.
[0121] Further, as the second coil 84a is arranged separately from
the main coil 56, the electric resistance measurement circuit 84b
can measure the electric resistance of the second coil 84a. Thus,
the main control circuit 101 can acquire the measured value R.
[0122] The second coil 84a is arranged in the area S1 which faces
the auxiliary heating plate 69 but does not face the main coil 56.
Compared to a second coil located in an area facing the main coil
56, it is possible to measure the electric resistance of the second
coil 84a accurately because the second coil 84a is less likely to
be affected by a large magnetic force of the main coil 56.
[0123] The second coil 84a is arranged to face the end portion 69c
(a portion adjacent to the facing area 69a) of the auxiliary
heating plate 69 across the fixing belt 50. According to this
arrangement, the second coil unit 84 can measure the electric
resistance of the second coil 84a at a location having a
temperature change identical to that of the facing area 69a (a
location having a correlation with the temperature change of the
facing area 69a).
[0124] Further, the second coil 84a is arranged to face at least
the paper passing area in the belt width direction. According to
this arrangement, the second coil unit 84 can measure the electric
resistance of the second coil 84a separately from the non-paper
passing area. Thus, the main control circuit 101 can acquire the
measured value R separately from the non-paper passing area.
Second Embodiment
[0125] Next, a second embodiment is described with reference to
FIG. 8. Here, components identical to those in the first embodiment
are described with the same reference numerals and the description
thereof is not provided.
[0126] FIG. 8 is a side view of a fixing apparatus 234 according to
the second embodiment. Further, FIG. 8 corresponds to the side view
of FIG. 6.
[0127] As shown in FIG. 8, the fixing apparatus 234 according to
the second embodiment does not include the second coil 84a of the
first embodiment. The fixing apparatus 234 according to the second
embodiment is different from the first embodiment in that it
includes a measurement unit 284 using the main coil 56. Further, a
reference numeral 284b indicates the electric resistance
measurement circuit in FIG. 8.
[0128] The IH coil unit 52 includes the main coil 56 (coil) which
heats the heating layer 50a through an electromagnetic induction.
The IH coil unit 52 also functions as the measurement unit 284. The
measurement unit 284 generates a magnetic field passing through the
auxiliary heating plate 69 through the energization to the main
coil 56. The measurement unit 284 measures the electric resistance
of the main coil 56.
[0129] The magnetic flux generated by the main coil 56 forms the
first magnetic path 81 and the second magnetic path 82. The
electric resistance of the main coil 56 varies in accordance with
the change of magnetism of the auxiliary heating plate 69.
[0130] As the high frequency weak current flows into the main coil
56, the electric resistance of the main coil 56 can be
measured.
[0131] The electric resistance measurement circuit 284b measures
the electric resistance of the main coil 56. It is assumed in the
present embodiment that the electric resistance of the main coil 56
measured by the electric resistance measurement circuit 284b is
"the measured value R". The main control circuit 101 acquires the
measured value R from the electric resistance measurement circuit
284b.
[0132] The main control circuit 101 determines whether or not the
measured value R acquired is smaller than the threshold value Rt
(for example, 1".OMEGA.").
[0133] By determining whether or not the measured value R is
smaller than the threshold value Rt, it is possible to determine
the change of magnetism of the auxiliary heating plate 69 for the
following reasons.
[0134] When the measured value R is greater than the threshold
value Rt, the auxiliary heating plate 69 has ferromagnetism because
its temperature is lower than the Curie point thereof. When the
auxiliary heating plate 69 has ferromagnetism, the magnetic flux
generated by the main coil 56 forms the first magnetic path 81 and
the second magnetic path 82.
[0135] On the other hand, when the measured value R is smaller than
the threshold value Rt, the auxiliary heating plate 69 has
paramagnetism because its temperature is higher than the Curie
point thereof. In such a case, the second magnetic path 82 is not
formed.
[0136] Thus, by determining whether or not the measured value R is
smaller than the threshold value Rt, it is possible to estimate the
magnetism of the auxiliary heating plate 69.
[0137] The main control circuit 101 controls the IH coil unit 52 to
reduce the heat generation through the electromagnetic induction
heating when the acquired measured value R is smaller than the
threshold value Rt.
[0138] In accordance with the second embodiment, the same effects
as the first embodiment can be obtained.
[0139] Further, compared with the case in which the second coil 84a
faces the end portion 69c of the auxiliary heating plate 69 across
the fixing belt 50, it is possible to measure the electric
resistance of the main coil 56 at a location facing the facing area
69a. Consequently, it is possible to determine the change of
magnetism of the facing area 69a.
[0140] Further, it is possible to measure the electric resistance
of the main coil 56 at the timing when the IH coil unit 52 does not
generate heat. For example, it is possible to measure the electric
resistance of the main coil 56 at a timing between print
operations, except for during the continuous paper passing and the
warming up (for example, when every 10 papers pass). As a result,
the change of magnetism of the facing area 69a can be determined
between print jobs.
[0141] Compared with the case in which the second coil 84a is
arranged separately from the main coil 56, the number of components
can be reduced and thereby the configuration of the fixing
apparatus 234 can be simplified.
[0142] Further, the electric resistance of the main coil 56 may be
measured at the timing when the IH coil unit 52 generates heat. For
example, the electric resistance of the main coil 56 is measured
during the continuous paper passing and the warming up. In this
way, it is possible to measure the electric resistance of the main
coil 56 at the location facing the facing area 69a in real time.
Consequently, during the continuous paper passing and the warming
up, it is possible to determine the change of magnetism of the
facing area 69a in real time.
Third Embodiment
[0143] Next, a third embodiment is described with reference to FIG.
9. Here, components identical to those in the first embodiment are
described with the same reference numerals and the description
thereof is not provided.
[0144] FIG. 9 is a side view of a fixing apparatus 334 according to
the third embodiment. Further, FIG. 9 corresponds to the side view
of FIG. 6.
[0145] As shown in FIG. 9, the fixing apparatus 334 according to
the third embodiment does not include the second coil 84a of the
first embodiment. The fixing apparatus 334 according to the third
embodiment is different from the first embodiment in that it
includes a second coil 384a arranged at the inner peripheral side
of the fixing belt 50. The second coil 384a is arranged at the
inner side in the diameter direction of the auxiliary heating plate
69. Further, a reference numeral 384 indicates the second coil unit
and a reference numeral 384b indicates the electric resistance
measurement circuit in FIG. 9.
[0146] The magnetic flux generated by the second coil 384a forms a
fifth magnetic path 87 that extends through the auxiliary heating
plate 69 before the auxiliary heating plate 69 loses its magnetism
due to the temperature thereof exceeding the Curie point thereof.
The fifth magnetic path 87 passes through the auxiliary heating
plate 69 in such a manner that it does not extend to the outer side
of the auxiliary heating plate 69 in the belt diameter
direction.
[0147] The magnetic flux generated by the second coil 384a forms a
sixth magnetic path 88 that extends through the heating layer 50a
of the fixing belt 50 when the auxiliary heating plate loses its
magnetism due to the temperature thereof exceeding the Curie point
thereof. The sixth magnetic path 88 extends to the outer side of
the auxiliary heating plate 69 in the belt diameter direction,
passing through the heating layer 50a. The electric resistance of
the second coil 384a varies in accordance with the change of
magnetism of the auxiliary heating plate 69.
[0148] By causing a high frequency weak current in the second coil
384a, it is possible to measure the electric resistance of the
second coil 384a. The electric resistance measurement circuit 384b
measures the electric resistance of the second coil 384a. It is
assumed in the present embodiment that the electric resistance of
the second coil 384a measured by the electric resistance
measurement circuit 384b is a "measured value R". The main control
circuit 101 acquires the measured value R from the electric
resistance measurement circuit 384b.
[0149] The main control circuit 101 determines whether or not the
acquired measured value R is smaller than the threshold value Rt
(for example, 1 "Q").
[0150] By determining whether or not the measured value R is
smaller than the threshold value Rt, it is possible to determine
the magnetism of the auxiliary heating plate 69 for the following
reasons.
[0151] When the measured value R is greater than the threshold
value Rt, the auxiliary heating plate 69 has ferromagnetism because
its temperature is lower than the Curie point thereof. When the
auxiliary heating plate 69 exhibits ferromagnetism, the magnetic
flux generated by the second coil 384a forms the fifth magnetic
path 87.
[0152] On the other hand, when the measured value R is smaller than
the threshold value Rt, the auxiliary heating plate 69 has
paramagnetism because its temperature is higher than the Curie
point thereof. In such a case, the magnetic flux generated by the
second coil 384a forms the sixth magnetic path 88, but the fifth
magnetic path 87 is not formed.
[0153] It is possible to estimate the magnetism of the auxiliary
heating plate 69 by determining whether or not the measured value R
is smaller than the threshold value Rt.
[0154] The main control circuit 101 controls the IH coil unit 52 to
reduce the heat generation through the electromagnetic induction
heating when the acquired measured value R is smaller than the
threshold value Rt.
[0155] In accordance with the third embodiment, the same effects as
the first embodiment can be obtained.
[0156] Further, in the present embodiment, the second coil 384a is
arranged at the inner side in the belt diameter direction of the
auxiliary heating plate 69 on the inner peripheral side of the
fixing belt 50. Compared with a second coil disposed on the outer
peripheral side of the fixing belt 50, it is possible to aggregate
the second coil 384a as well as the auxiliary heating plate 69 on
the inner peripheral side of the fixing belt 50.
[0157] In accordance with the fixing apparatus of at least one
embodiment described above, the excessive temperature rise of the
IGBT element 68a can be suppressed, which can prevent the IGBT
element 68a from being damaged.
[0158] Further, the heating layer 50a may be made from the magnetic
material such as nickel.
[0159] Furthermore, the measurement unit described above is not
limited to the electric resistance measurement unit described
above. For example, the measurement unit may include a temperature
measurement unit which measures the temperature of the auxiliary
heating plate 69. For example, the temperature measurement unit is
a temperature sensor. By measuring the temperature of the auxiliary
heating plate 69, it is possible to determine whether or not the
temperature of the auxiliary heating plate 69 exceeds the curie
point directly. That is, as long as the measurement unit can
measure the state of the auxiliary heating plate 69, no limitation
is given to the configuration of the measurement unit.
[0160] Further, the present invention is not limited to that the
main control circuit 101 indirectly determines whether or not the
temperature of the auxiliary heating plate 69 exceeds the Curie
point based on the measured results by the electric resistance
measurement circuit. For example, the main control circuit 101 may
determine whether or not the temperature of the auxiliary heating
plate 69 exceeds the Curie point directly based on the measured
results by the temperature sensor. That is, as long as the main
control circuit 101 can control to reduce the heat generation by
the IH coil unit when it is determined that the temperature of the
auxiliary heating plate 69 exceeds the Curie point based on the
measured results by the measurement unit, no limitation is given to
the determination method.
[0161] The functions of the fixing apparatuses in the embodiments
described above may be realized by a computer. In that case,
programs for achieving the functions may be recorded in a
computer-readable recording medium, and the programs recorded in
the recording medium may be read by a computer system and executed
to realize the functions. Further, it is assumed that the "computer
system" includes hardware such as an OS, a peripheral machine and
the like. Further, the "readable recording medium" refers to a
movable medium such as a flexible disc, a magnetic optical disc, an
ROM, a CD-ROM and the like, and a storage device such as a hard
disk arranged inside the computer system. Further, the
"computer-readable recording medium" may store programs dynamically
for a short time like a communication line in a case of sending the
programs via a network such as the Internet, a telecommunication
line such as telephone line and the like, and may also store
programs for a certain time like a volatile memory inside a
computer system consisting of a server and a client in that case.
Further, the programs may be used to realize part of the
aforementioned functions, or may also be used to realize the
aforementioned functions through a combination with the programs
that have already stored in the computer system.
[0162] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the invention. Indeed, the novel
embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the embodiments described herein may be made without
departing from the spirit of the invention. The accompanying claims
and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
invention.
* * * * *